In wireless data communication systems, such as wireless LANs, the same data reaches wireless terminals that are present within an area receiving the radio waves from a wireless relay station. That is to say, communication data between the wireless relay station and a given wireless terminal is simultaneously communicated to the other terminals as well. Consequently, it is not possible that a plurality of wireless terminals simultaneously perform communication completely independently from one another. Therefore, when one wireless terminal starts to send a large amount of data, the other wireless terminals will be influenced by this communication. For example, effects such as a slow-down of the communication speed or delays in the response may occur. In VoIP (voice over IP) technology, which is used in IP (internet protocol) telephones, audio data is communicated at predetermined intervals between a wireless relay station and a wireless terminal. In this situation, when a communication necessitating a lot of communication bandwidth is started by another wireless terminal, the audio data being communicated may arrive at a certain delay. As a result, the audio of the communication partner may be reproduced at a delay, or a cut-off noise or the like may occur when the reproduction of the audio data occurs too late. For this reason, there is a need for assigning priorities to certain communications when a plurality of communications are carried out. The following is an explanation of methods for priority control that have been proposed up to now.
IEEE 802.11 is an international standard for wireless LANs. There are several standards, depending on differences in the radio bands and the modulation method used. For example, the standard for wireless LANs that are presently in wide use is IEEE 802.11b, which allows communication at a maximum of 11 Mbps in the 2.4 GHz band. This wireless LAN standard has two communication modes.
One communication mode is a mode called DCF (distributed coordination function). In this mode, when no communication is performed, a wireless terminal that wants to send data can send the data. When a plurality of wireless terminals attempt to send data simultaneously, there is the possibility of conflict among these communications, so that in the period after the previous communication has finished and until the next communication starts, the standard mandates that each wireless terminal performs the sending of data after waiting for a random standby time (referred to as “backoff time” below). Therefore, when there are a plurality of wireless terminals, the wireless terminals are each given an equal opportunity to transmit, and no priority is given to any of the wireless terminals.
Another communication mode is the mode referred to as PCF (point coordination function). In this mode, the wireless relay station can intentionally secure the communication bandwidth for certain wireless terminals periodically. However, currently available wireless LAN systems do not support the PCF mode, and at present, priority control in wireless LANs is not performed by a standardized method.
Recently, EDCF (enhanced DCF), which is an expansion of DCF, has been considered as a part of the 802.11e standard. Whereas DCF provides only one send queue, EDCF provides a maximum of eight send queues. It has been proposed to differentiate these send queues in accordance with the priority degree of the communication data. With such a scheme, when send data is stored in send queues with different degrees of priority, then the backoff times are set shorter for queues with higher degrees of priority. Therefore, data with a high degree of priority is more likely to be sent.
Another proposal has been to prioritize certain communications by intentionally shortening the time that has to be waited after other communication has finished, when communicating in the DCF mode. This communication scheme operates without waiting for the backoff time that is stipulated in the aforementioned 802.11 standard, but compatibility is preserved, because it operates without problems even when operating simultaneously with devices conforming to the 802.11 standard. However, as the number of wireless terminals operating in this manner is increased, there is the danger that the probability for contention rises, and the communication quality decreases. Consequently, this scheme is useful only when the number of prioritized communication terminals is small.
The above-described prioritization methods are all methods performing the priority control separately for each wireless terminal, that is, separately for each communication. By contrast, the following methods, for example, are methods in which the priority control is performed depending on the data type to be communicated:    (a) Method using the value set for the layer-2 service class (COS) in the user priority bits stipulated under 802.1Q;    (b) Method using IP priority order/DSCP (differential service code bit) in the type-of-service (TOS) byte of the IPv4 header.
Both COS and TOS consist of three bits. It is possible to prioritize the packets of a specific data type by determining a common packet priority order and referencing this information with the wireless terminals or the data relay device. At present, as IP telephone equipment is developed, this approach is adopted most often. However, in general there are only few devices that utilize this information, and there is the risk that difficulties arise when mixing different services. There is furthermore the possibility that the entire network is put at risk when there are terminals with which an application operating on wireless terminals and wired terminals is intentionally set to incorrect settings in order to ensure a high priority degree.
Furthermore, JP 2003-134077A describes a method for sending videos with MPEG video encoding, in which an MPEG header is referenced and data is distributed over a plurality of queues with different priority degrees. However, the algorithm for referencing the header and distribution to the queues is fixed, so that it is difficult to dynamically change the method of the priority control.
As described above, several methods for performing priority control on a wireless LAN have been proposed. However, ordinarily the communication bandwidth of wireless LANs is narrower than that of wired networks. For this reason, even when priority control is performed, it is conceivable that most of the available bandwidth of the wireless network is used up by prioritized communication. In this case, there is the possibility that it is not possible to preserve the communication quality of prioritized communication, even when performing priority control, due to the influence that prioritized communications have on one another on a crowded network.
In order to perform stream communication as in VoIP over a wireless LAN, periodic communication must be realized reliably. There are the following four problems regarding this:    (1) Priority control in wireless communication zones;    (2) Distribution of prioritized packets at wireless terminals and relay device;    (3) Optimization of the method for determining which communication should be prioritized;    (4) Bandwidth management of prioritized communication at the wireless relay station.
Of these problems, problem (1) is realized by the above-described methods. Regarding the problems (2) and (3), it is desirable that it is possible to switch flexibly, in accordance with the actual needs, for example between priority control at the wireless terminal level, priority control at the application level or priority control according to data type. For example, it is preferable to give priority to IP telephony over internet radio audio streams, even though both are streamed audio. Moreover, it is desirable to enable settings that give priority to IP phone calls by customers who pay for charge over IP phone calls by customers who don't pay.
As for problem (4), there is a limitation on the bandwidth that can be used by wireless terminals performing prioritized communication, so that one problem that arises is how conflicts among prioritized communications can be solved. Moreover, in wireless LANs, when the state of the wireless communication between the wireless terminals and the wireless relay station becomes poor and errors become frequent, then a function is activated that reduces the connection speed to maintain the communication quality. When the connection speed is lowered and the transferred data amount per unit time is kept the same, then the consumption bandwidth is effectively broadened. Thus, it becomes necessary to monitor the connection speed in the bandwidth management.
Furthermore, when there are a plurality of wireless relay stations, it may occur that a wireless terminal is switched automatically, as a result of being moved, from a wireless relay station to which it was connected previously to another nearby wireless relay station. In this case, it is expected that the prioritized communication that was already realized continues seamlessly even after the switch to another relay station. Therefore, it is necessary to reserve a certain communication bandwidth at the wireless relay station to which a wireless terminal is moved from another wireless relay station. All of the above problems need to be considered for the bandwidth management of prioritized communications.
It is an object of the present invention to realize stable wireless communication by priority control of the communication of wireless terminals.